Hi Gert
You have already discovered that the stable version (3.8) only runs on older Ubuntu versions. V4 was released as a beta release last year, and runs on at least 18.04, and probably 20.04. Have a look at
https://grasehotspot.org/documentation/v4-nightly-builds for how to get started with the V4 beta builds.
I have no intention of shutting down the apt repo's, or letting the domain expire (the risk for a squatter to get the domain and to try and do something malicious is too high). If I have the time in the future then I'll keep working on V4, and updating it for Ubuntu 20.04 and future LTS versions.
Regards
Tim